Oil exploration companies have begun to drill for oil in a mile or more of water in Canada, Australia, Norway, the United Kingdom, the USA and other countries. More drilling sites are located where the sea is very deep and because the distance to be drilled into the seabed is greater, the time required to perform the drilling is increasing. With increased drilling time at greater sea depths, the chances of a blow-out whereby the oil escapes from an undersea reservoir also increases.
An example of the trend would be the deepwater drilling being undertaken in the Gulf of Mexico. The company Chevron completed their Perdido Development in March of 2010 and is tethered in nearly 8,000 feet of water whereas, the infamous BP Macondo well that incurred a blow out, was tethered in 5,000 feet of water.
It is very difficult to deal with a blow out in very deep water because of the remoteness from the surface, the high hydraulic pressures in the oil reservoirs and the limited experience with these types of situations.
The major exposed elements of an undersea oil well include among others:                A large diameter steel casing that is drilled or driven into place on the sea bed and solidly cemented to the floor of the ocean.        The well-head is a large steel fitting that sits on the ocean floor and is attached to the steel casing.        
Atop the well-head is a Blow Out Preventer (BOP) connected to the flanged discharge connection of the well-head. A BOP may contain several closing devices called ‘rams’ that serve different functions: for example cutting drill pipe or casing or sealing around drill pipe while leaving it intact. A BOP may also contain annular preventers that are hard rubber devices in the shape of a tire that is designed to expand and seal around drill pipe or else seal the well entirely if drill pipe is not present.
A production riser/pipe transfers the oil flow from the discharge flange of the BOP to a surface vessel. This top portion of the BOP stack is called the lower marine riser package (LMRP).
The first stage of defence against a blow out is proper drilling techniques and an accurate control of the mud density and close tracking of its return flow rate. The principal second stage of defence to prevent a blow out is the Blow Out Preventer or BOP. If there is a problem of a potential blow out at any time during the drilling the only defence or shut off between the oil reservoir located deep under the seabed and the drill ship located at great distance above the seabed is the BOP.
During drilling operations the drill pipe which has the drill bit at its end passes through the riser production pipe and the BOP and then down through the well head to the bottom of the oil well. As the well is drilled ‘mud’ is forced through a drill pipe to the drill bit and returns up the well bore in the ring shaped void or annulus between the outside of the drill pipe and the production pipe. The column of drill mud exerts downward hydrostatic pressure to counter opposing pressure from the formation being drilled. When a ‘kick’ (influx of formation fluid) occurs, rig operators or automatic systems close the BOP units to stop the flow of fluids out of the well bore.
The BOP is designed to rapidly seal of the space or annulus between the riser pipe and the drill pipe and if necessary to cut through the drill pipe and close off all possible flow. As mentioned, when the blow out begins there is normally a column of heavy mud in the production tubing and the weight of this column helps to slow down the rate of discharge of oil from the reservoir. However as gas or lighter oil replace this mud there is less resistance to vertical discharge and the velocity of the discharging material will continue to accelerate until a steady-state condition of free discharge is achieved. At this point, the material in the entire length of production tubing between the oil reserve and point of discharge is travelling at a velocity that is proportional to the pressure in the oil reservoir and contains a quantity of energy in the form of momentum.
The BOP is a sophisticated electro-mechanical device that includes multiple stages of sealing devices. Its purpose is to stop the uncontrolled discharge of gas and oil and sediment from the oil reservoir. In the early days of oil explorations, before the advent of the BOP, oil geysers or blow-outs were a common occurrence.
Without the sealing mechanism of the BOP, oil would simply flow up through the production tubing and riser piping to the surface vessel at an uncontrolled rate. The rate of discharge will be a function of the pressure in the oil reservoir minus any piping friction losses incurred between the entrance to the production tubing and the point of discharge. If the discharge is flowing through a malfunctioning BOP, it may provide some additional pressure losses particularly if it's closing mechanisms had partially functioned.
Once a situation of free discharge has been achieved the rapid closure of any of the multiple-staged throttling devices in the BOP may result in severe and even destructive stresses on the well head and production casing in the form of water hammer. In the petroleum industry jargon they refer to this as shocking the well.
This invention is designed to provide a solution for shutting down an oil well that has lost the operation of its BOP and is now discharging freely at some point between the riser of the production casing and the surface vessel. If the BOP is not operational it is quite possible that it has incurred some physical damage and will probably, but not necessarily, require its removal prior to beginning the procedure for bringing the well under flow control. Depending upon the cause of the accident and the nature and velocity of the material discharging from the oil well the riser piping may have incurred considerable stresses and erosion that weaken its structural integrity. As such, any solution will require that the minimum amount of stress be applied to the remaining piping and connections.
It is important to understand the basic principles of the problem to understand the development of the philosophy of the solution. Essentially, the problem is the energy in the discharging jet of oil. The energy is generated by the hydrostatic pressure in the oil reservoir and conversely the velocity of the discharging oil jet as the pressure (potential) energy converts into velocity (kinetic) energy. If the pressure in the oil reservoir was less than the pressure of the seawater at well head there would be no flow out of the reservoir. In the Gulf of Mexico, where the water has high levels of dissolved solids, each foot of water equals 0.465 psi.
Accordingly at a depth of 5,000 feet the pressure of the seawater will be approximately 2,325 psi. If the pressure of the oil discharge at 5,000 feet of depth was 2,325 psi, there would be no flow and remotely controlled vehicles would simple bolt or fix an attachment over the open riser piping to stop all possible flow.
However the pressure in an oil reservoir can reach 30,000 psi. As the oil flows up the production tubing between the reservoir and the well head there will be some friction losses and loss of energy from the flow stream but substantial energy will remain and as mentioned is converted into an equivalent amount of kinetic energy or velocity. This increase in kinetic energy and decrease in pressure follows the Bernoulli principle and the Law of conservation of energy.
Based on the Bernoulli principle, the kinetic energy of a flow stream can change by changing the area of the superficies of flow along a streamline. By increasing the surface area of the discharge of the oil we can reduce its velocity and hence the forces it will exert against an object placed in its path. Accordingly, it is possible to decrease the force required to place an object in a flow stream by either decreasing the velocity of the flow or by reducing the surface area of the object that confronts the flow stream.
At the point of discharge the pressure energy of the flow stream is entirely converted into velocity of the flow stream. A simple formula allows to calculate the velocity (V) if the pressure or head (H) is known and is H=V2/2 g or V=the square root of (2 gH) where g is the gravitational constant.
In order to place an object in a flow stream one needs to have sufficient force to hold it in place. Flow forces will cause unsupported or insufficiently supported equipment to be thrown aside. The challenge in holding an object in place in a deepwater situation arises from the fact that the working point is at the surface of the ocean that may be miles above. This implies that the shape of any objects inserted through the jet should have an orifice opening of sufficient size such that it can be positioned using the available positioning forces. These forces consist of the force available from an ROV or a surface ship or platform. By lowering a connection pipe from the surface vessel considerable vertical force can be applied and consists of the weight of the pipe and hydraulic loading from the surface vessel.
Overcoming the discharge velocity is the principle engineering challenge and any solution must be able to accommodate a wide range of discharge pressures up to 30,000 psi. Essentially, there is a need for a process whereby an apparatus can be assembled at great depth that has a configuration such that the positioning forces available are sufficient to overcome the forces of the discharge jet against the objects being placed. Without exceeding the point of equilibrium between the forces exerted by the discharge jet versus the available force for positioning the apparatus cannot be assembled.
The magnitude of these two forces will dictate the configuration of the components and evidently a higher flow velocity will dictate the design of components that present less resistance during their placement. The size of the riser piping and connections is fairly standard; the discharge pressure from the oil reservoir is not standard. As such, the solution and its methodology remains the same but the size and configuration will reflect the equilibrium sought between the two opposing forces and will be largely a function of the oil flow velocity. In general, the installation of one discharge orifice or multiple discharge orifices are required at the point of free discharge when a reduced flowing velocity is required to place equipment in the flow stream.
A recent deepwater oil spill for which this invention can be applied is the BP Deepwater Horizon drilling of the Macondo well in the Gulf of Mexico.
A National Commission was established by President Obama to investigate. It clearly establishes the great numbers and effort by BP, the oil industry, government scientists and independent engineering consultants that spent months researching a suitable technique to shut down this oil spill. The effort by BP to partially contain the oil lost was an unsatisfactory solution. The BP attempts to shut down the well by obstructing the flow through the production tubing was totally unsuccessful given the high discharge pressure of the Macondo well. As for the BP position, they indicate on the home page of their BP Global website: ‘This was a situation never encountered before and required a number of solutions that were new to BP and the industry’.
The complete National Commission report is available at the following web site: http://www.oilspillcommission.gov/final-report
Alternatively, the staff working paper No. 6 prepared by the New York Times is available at: http://graphics8.nytimes.com/packages/pdf/science/Containment.pdf
There was some early consideration given to installing a second BOP at the Macondo well but it was openly admitted that the no one knew how to engineer its installation under the existing severe conditions. The best solution at the time was to increase the containment of the leaking oil by improving the seal between the containment dome and the oil well riser piping and eventually to attempt a bottom kill by injecting mud and concrete through two relief wells.
The intent of this invention is to provide an apparatus and a sequential methodology and method for the design and execution so that free discharging oil risers can be brought under control using a throttling device.